Battery in a Photovoltaic Power Supply System - Standards

This overview on standards for batteries is an extract of the publication: GTZ, Division 44, Environmental Management, Water, Energy, Transport: Quality Standards for Solar Home Systems and Rural Health Power Supply. Photovoltaic Systems in Developing Countries, February 2000.

Comment: The storage batteries are still the weakest, most vulnerable component in a photovoltaic power supply system. This might also be the reason why different types of batteries, ranging from automotive starter batteries and so-called “Solar Batteries”, all the way to high-quality industrial tubular plate (OPZS) batteries, and also sealed maintenance-free batteries, are used in PV systems.

The “Universal Standard for SHS” [1] gives a brief overview of the various aspects, advantages and disadvantages of the different battery types and their useful application in SHS. Some of the following observations may serve as an introduction for planners of subsequent specifications:

The most important feature of battery operation in SHSs is cycling. During the daily cycle, the battery is charged over the day and discharged by the night-time load. Superimposed onto the daily cycle is the seasonal cycle, which is associated with periods of reduced radiation availability. This, together with other operating parameters (ambient temperature, current, voltage, etc.), affects the battery life and maintenance requirements. In order to maximise the lifetime of lead-acid batteries, the following operating conditions must be avoided:

• High voltage during charging (to prevent corrosion and loss of water)
• Low voltage during discharge (corrosion)
• Deep discharge (sulphation, growth of dentrites)
• Extended periods without full charging (sulphation)
• High battery temperature (all aging processes are accelerated)
• Stratification of the electrolyte (sulphation)
• Very low charge current (sulphation)

These rules lead to specifications for sizing (both battery and PV generator) and for battery protection procedures (charge regulator). However, it must be pointed out that some of the rules generally contradict each other (e.g. full charging requires high voltages but high voltages accelerate corrosion), so compromises must be found that take the particular local conditions into account: solar radiation, PV module and battery prices, duties and taxes, local manufacturing, recycling infrastructure, etc. Perhaps this explains the lack of consensus on this issue among the various sources of information (standards, experts, etc.) that have been consulted during the preparation of this standard; therefore, the requirements given below should be adapted to suit the local circumstances.

The need to prevent excessive discharge leads to the need to limit the maximum depth of discharge to a certain value, PD_MAX, which usually ranges from 0.3 to 0.6, but can approach 0.8, depending on the type of battery. The supply to the load must be cut off when this limit is reached. The available or useful capacity, C_U, is therefore less than the nominal capacity, C_B (which refers to the whole charge that could be extracted from the battery if no particular limitations were imposed) and equal to the product C_B * PD_MAX, such that:

C_U = C_B * PD_MAX

A good compromise between cost and reliability is typically obtained with a battery whose useful capacity ranges from three (in regions where extended cloudy periods are not expected) to five (in regions where cloudy periods are expected) times the total daily energy consumption in the house, so that the depth of discharge in the daily cycle, PD_d ranges from 0.06 to 0.2. The selection of a particular capacity mainly depends on the battery type. High-quality batteries are better able to resist deeper cycling than low-quality batteries. Hence, for the same application, high-quality batteries can be smaller than low-quality batteries, in terms of nominal capacity.

The highest-quality PV batteries are made with tubular plates and grids with low Sb-Se content. More than 8 years life, with PD_d = 0.2 and a maintenance period of 1 or 2 times per year, are attainable with such batteries. A particular disadvantage of tubular batteries for SHSs is that they do not readily accept low rates of charge. They are also expensive and are rarely available in the current markets in developing countries. Nevertheless, they should not be excluded from SHS programmes. On the contrary, it is recommended that large rural electrification programmes consider encouraging manufacturers to put these products on the market.

In contrast, automotive batteries, usually referred to as SLI, have a number of advantages. They are usually the cheapest batteries when compared in terms of nominal capacity (the difference in cost can be 4 or 5 fold), they are often locally produced and are widely available. Local production is not only convenient for economic and social reasons, but also because it represents the best means of recycling old batteries and avoiding environmental damage. Their main drawback lies in their relatively short lifetime. Because their cell design is optimised to deliver heavy currents during short periods of time, they have large areas of thin plates, and are poorly suited to supplying smaller currents for many hours before being recharged, as is required by SHS. It is therefore necessary to use larger battery capacities leading to PD_d £ 0.1, and a density of electrolyte which is lower than would normally be used in this type of battery (for example, 1.24 instead of 1.28 g/cl). This is necessary to reduce grid corrosion and hence to lengthen battery life. The associated increase in internal resistance in the battery does not pose any problems in SHS, because the charge and discharge currents are relatively low by comparison to conventional battery charge and discharge regimes. Classical SLI batteries use lead grids alloyed with antimony and require periodic topping up with water.

The short lifetimes of automotive batteries can also be compensated to some extent by introducing relatively simple modifications to the battery design but not to its technology. The most common modifications are thicker electrode plates and a larger quantity of acid solution in the space above the plates. Such modified SLI batteries are sometimes marketed as "solar" batteries and represent a promising alternative for the future of SHSs. Wherever possible, modified SLI batteries should be selected (and local manufacturers should be encouraged to make them) in preference to conventional SLI batteries. Certain conditions must be met in order for a battery to be categorised as "modified SLI", as follows:

• The thickness of each plate must exceed 2 mm.
• The amount of electrolyte must exceed 1.15 l per 100 Ah of 20-hour nominal capacity and per cell.
• The separator must be made of microporous polyethylene.
• The density of electrolyte must not exceed 1.25 g/cl.

"Low-Maintenance" SLI batteries, sometimes marketed as maintenance-free batteries, often employ grids containing calcium alloys. The calcium increases the voltage at which gassing begins but reduces the cohesion of the active material to the grids. Hence, it cuts down the loss of water but also reduces the cycle life. Such batteries are particularly vulnerable to damage from deep discharge. In addition, they are also liable to be damaged by high temperature variations. Hence, many PV system designers strongly recommend that they not be used in PV applications in hot countries. However, the maintenance-free feature is still attractive, and extensive use has been made of these batteries in some countries like Brazil.

"No-maintenance" batteries of various kinds are also made for professional applications by using a semi-solid electrolyte (gel or malting). Such batteries, referred to as VRLA (valve-regulated lead acid), are more often resistant to deep discharges, but they are usually very expensive for SHSs, and they require specific recycling facilities. They are not considered in the present standard although they represent a legitimate technology choice in some cases. The same is valid for NiCd batteries.

The 20-hour nominal battery capacity in amp-hours (measured at 20 W and up to a voltage of 1.8 V/cell) should not exceed CR times the PV generator short-circuit current in amps (measured at Standard Test Conditions). CR values are proposed for each type of battery in the table below:

Battery type	CR
Tubular	15-20
SLI (automotive): - Classical	30-40
- Modified	35-40
- Low-Maintenance	30-40

The maximum depth of discharge, PD_MAX (referred to as the 20-hour nominal battery capacity) should not exceed the values proposed in the table below:

Battery Type	PDMAX
Tubular	70-80
SLI (automotive): - Classical	30-50
- Modified	40-60
- Low-Maintenance	20-30

The useful capacity of the battery, C_U (20 hours nominal capacity, as defined above, multiplied by the maximum depth of discharge) should allow for a three to five-day period of autonomy.

The cycle life of the battery (i.e., before its residual life drops below 80% of the nominal capacity) at 25°C must exceed NOC cycles when discharged down to a depth of discharge of 50%. A NOC value is given for each type of battery in the table below.

Battery Type	NOC
Tubular	600
SLI (automotive) - Classical	200
- Modified	200
- Low Maintenance	300

Comment: As the discussion of the "Universal Standard" above already shows, selecting the “right” battery type for PV systems is a difficult task, considering all of the different aspects of cost, lifetime, local availability, maintenance, recycling, etc. Another fact is, that no international standards for type-testing of batteries for PV applications are available as yet.

The IEC standards 60896 Part 1 and 2 “Stationary lead-acid batteries - General requirements and methods of test. Part 1: Vented types, Part 2: Valve-regulated types” give the general test methods for stationary batteries, but also include the comment that special test procedures for PV applications will be worked out by IEC group TC 21/TC 82. As confirmed by IEC, Geneva, a new standard on solar batteries is still being written and will have the number IEC 61147.

Up to now, the only standard available on solar batteries is the French standard NF C58- 510 “Lead-acid secondary batteries for storing photovoltaically generated electrical energy”, which will be used temporarily by PV GAP and the IEC SHS standardisation group. Therefore, the type-test procedures described in this standard will be the basis of the following battery specifications:

In addition to the above-mentioned standardisation activities of IEC, the CENELEC committee BTTF 86-2 has also drafted a standard proposal entitled "Accumulators for Use in Photovoltaic Systems, Safety-Test Requirements and Procedures", which was kindly made available by TÜV-Rheinland. As this proposal is not yet complete (no cycle tests, etc.) and all tests described here are also included in the French NF C58-510 standard, this standard-proposal is not considered in the following specifications. 

The rechargeable battery shall consist of one 12 V_DC vented type lead-acid “solar” battery.

Optional: The rechargeable battery shall consist of one 12 V_DC valve-regulated type maintenance-free lead-acid battery.

Health: The rechargeable battery shall consist of a 12 V_DC (24 V_DC) vented-type “heavyduty” tubular lead-acid battery.

The battery must be type-tested and certified in accordance with NF C 58-510 “Lead acid secondary batteries for storing photovoltaically generated electrical energy”, and/or IEC 60896-1 or -2 “Stationary lead-acid batteries - General requirements and methods of test. Part 1: Vented types, Part 2: Valve-regulated types” (will be replaced by IEC 61147).

The following tests must be performed, documented and certified as described in NF C58-510:

• Nominal capacity Cn in Ah (generally C10)
• Rated capacity Ct. (10-hour capacity C10 , 20-hour capacity C20 or 100-hour capacity C100 given by the supplier
• Number of cycles in a constant average state of charge (DOD=40%) Minimum requirement: 400 cycles
• Number of cycles in a changing average state of charge (DOD=20%) Minimum requirements: 1500 cycles for vented tubular plates, 900 cycles for sealed and vented flat plates
• Suitability for operation under increasing and decreasing “state of charge” conditions. Minimum requirements: 95% of C10 after 60 cycles
• Suitability for overcharging for 400 days at 2.35 V/cell. Minimum requirements: Vented cells - no dangerous gases escape from the cells; valve-regulated cells: recombination of oxygen and hydrogen > 95%
• Ampere-hour efficiency at discharge until 0.75 C100 Minimum requirement: not given.
• Enclosure test: 4 hours at 65° C and alternating temperature 30° C without any deformation
• Cell-sealing test. Minimum requirement: No seepage of electrolyte at an inclination of 30° and under a pressure of 0.1 bar
• Vent plug efficiency test. Minimum requirement: During the overcharge test no sulphuric acid and no explosive concentration of hydrogen escapes from the cells.
• Drop resistance test. Minimum requirement: 10 cm drop with all edges on concrete

 

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